Typical bar code readers produce light beam scanning patterns by oscillating at least one mirror to sweep or scan the light beam along a line, or within an area when combined with other mirrors. One configuration which is typically used to oscillate a mirror is a resiliently mounted shaft which supports the mirror. An electromagnetic arrangement is coupled to the shaft to produce the power required to oscillate the shaft about an axis within a predetermined range of angles. These oscillators (resonant motors) are designed to have a resonant (natural) frequency substantially at the frequency desired for scanning. This design practice greatly reduces the power required to oscillate the mirror.
Conventional drive circuits for resonant motors allow theses motors to oscillate at a resonant frequency by using a feedback loop. The amplitude of such oscillation is controlled by limiting the maximum level of the oscillation drive voltage to limit power to the motor. Normally, the drive voltage is below the supply voltage of the drive circuit and a substantial amount of the power required to drive the motor is dissipated in the operational amplifiers and transistors which are part of the motor drive circuit. Thus, the efficiency of many resonant motor drive circuits is not maximized, and may be as low as 50 percent.
As scanning speeds for bar code readers increase, the power requirements for the resonant motors of such readers have also increased. With this increase in power requirements, the power required to drive the resonant motors has become a significant portion of the total power and current required by the reader. As a result, the cost of the circuitry for readers has increased due to the increased complexity and current carrying capacity of the circuitry.
In addition to power consumption problems, conventional drive circuits are problematic in that they may cause a resonant motor to oscillate at resonant frequencies other than the desired resonant frequency. This problem is a more serious concern when complex spring assemblies are used to resiliently support a motor shaft.
In view of the problems discussed above and other problems which arise as a result of using resonant motors with feedback driven drive circuits, it would be desirable to provide a resonant motor drive circuit which uses switching circuits to reduce power consumption and includes circuitry configured to initiate oscillation of a resonant motor at the proper resonant frequency.